1. Technical Field of the Invention
The present invention relates to a phase synchronization system which follows the frequency and the phase of an input signal.
2. Prior Art and Its Problem
Heretofore a phase synchronization circuit of the PLL (Phase Locked Loop) system has widely been used, for example, in a carrier recovery circuit for creating a reference carrier necessary for demodulating a phase-modulated signal and a frequency tracking circuit for tracking a certain electric wave. The frequency and phase pull-in characteristics of the phase synchronization circuit of the PLL system depend on an equivalent noise band width (a loop band width) which is determined by a loop gain and characteristics of a loop filter and a phase comparator. An increase in the loop band width will quicken the occurrence of the pull-in operation and a decrease in the loop band width will defer the occurrence of the pull-in operation. However, when the loop band width is large, an output phase jitter in the steady state is large, whereas when the loop band width is narrow the jitter is small.
In general, a phase synchronization circuit is required to have a quick pull-in characteristic and to be small in the phase jitter in the steady state, but these two requirements are contradictory to each other, as mentioned above.
An example of the prior art for solving this problem has been employed in a carrier recovery circuit of the TDMA communication system. Since a TDMA signal is composed of a plurality of asynchronous burst signals, its demodulation calls for a demodulating operation which takes place while generating a reference carrier for each burst signal, and the carrier recovery circuit needs to be capable to establishing synchronization in a very short period of time.
Furthermore, since the TDMA signal is generally high in transmission rate, it is required that the phase jitter in the steady state be very small. To meet this requirement, the carrier recovery circuit is controlled so that in the pull-in state it increases the loop gain to widen the loop band to quicken the pull-in operation and in the steady state it decreases the loop gain to narrow the loop band to reduce the phase jitter.
FIG. 1 shows an example of the constitution of the above-said prior art. In FIG. 1(a) reference numeral 101 indicates a signal input terminal, 102 a phase comparator, 103 a VCO (Voltage-Controlled Oscillator), 104 an amplifier, 105 a loop filter, 106 a timing signal input terminal, and 107 an output terminal. FIG. 1(b) shows the phase comparison characteristic of the phase comparator 102, the abscissa representing the phase difference .theta. between the input signal and the output signal from the VCO 103 and the ordinate the output voltage.
The prior art example shown in FIG. 1 operates in the following manner. The phase comparator 102 outputs, as a voltage, the difference in phase between the input signal and the output signal from the VCO 103. This voltage is applied via the amplifier 104 and the loop filter 105 to the VCO 103, controlling it so that the frequency and the phase of its oscillation signal approach the frequency and the phase of the input signal. The amplifier 104 is to adjust the loop gain for changing the loop band width, depending on whether the circuit performs the pull-in or steady-state operation, as referred to previously. The amplification degree of the amplifier is controlled by a timing signal which is separately detected and applied to the terminal 106.
This system has a defect of needing, for changing the loop band width, timing information as to when the pull-in operation is to be started and when the steady state has been restored. This necessitates the use of a circuit for detecting the timing and generating the timing information, and hence introduces complexity in the apparatus.
On the other hand, the phase synchronization circuit of the PLL system suffers also the degradation of the phase pull-in characteristic which is commonly referred to as a hang-up phenomenon, and difficulty has been encountered in employing this circuit in a case where phase synchronization must be established in a short period of time.
The hang-up phenomenon refers to a state in which when the phase difference between the input signal 101 and the output signal of the VCO 103 at the terminal 107 is .pi., the output of the phase comparator 102 beciomes zero and the phase of oscillation of the VCO 103 undergioes no change, with the result that the phase difference settles into .pi., making it impossible to establish synchronization as will be seen from the phase comparison characteristic shown in FIG. 1(b). Moreover, when the phase difference is not exactly equal to .pi. but very close to .pi., a phenomenon occurs which can be regarded as the hang-up phenomenon; in this case, the output of the phase comparator 102 becomes close to zero and much time is needed for establishing synchronization.
A method which prevents this hang-up phenomenon and improves the pull-in characteristic is set forth in a literature entitled "Studies of Carrier Recovery Circuit for Use in Synchronous Demodulation of TDMA Signal" (Journal of Institute of Electronics and Communication Engineers of Japan, Vol. 54-B, No. 4, 1971, pp. 160-167). This method is called a kick off system, according to which the phase difference is measured at the start of synchronization and if the phase difference is close to .pi., the phase difference is forcibly shifted by .pi. from a hang-up region (near .pi.) to a stable region (near zero). This system also has the drawback that information on the starting timing for synchronization operation is needed for operation, as is the case with the system which employs different loop gains for the pull-in operation and the steady-state operation. Besides, according to this system, when the input signal contains noise, there are cases where the hang-up phenomenon cannot be detected or the noise is detected as the hang-up phenomenon; namely, this system suffers so-called nondetection and erroneous detection, and hence cannot yet completely eliminate the hang-up phenomenon.